NH4PO3-(NH4)(2)SiP4O13 composite, a potential electrolyte for intermediate temperature fuel cells that operated around 250 degrees C, was synthesized with a solid-state reaction method. Electromotive forces (emfs) as measured with hydrogen concentration cells showed that the composite was a pure proton conductor at hydrogen partial pressure from 10(2) to 10(5) Pa. Its proton transference numbers were determined to be 1.0 at 150 degrees C, 0.99 at 200 degrees C, and 0.99 at 250 degrees C. Fuel cells that used NH4PO3-(NH4)(2)SiP4O13 as electrolytes were also fabricated. Maximum power density was 6.6 mW/cm(2) at 250 degrees C when dry hydrogen and dry oxygen were used as the fuel and oxidant, respectively. Improved cell performance is expected by increasing cathode activity, increasing the electrolyte density, and decreasing the electrolyte thickness. (c) 2006 Elsevier B.V. All rights reserved.